I. Field of the Invention
The present invention relates generally to the field of medical diagnosis and specifically to a process of classifying a patient's functional status to assess the severity of the patient's disease. The disclosed method provides a more sensitive method that is easier to use than currently available classification systems. In addition, the present invention provides feedback during long-term follow-up in patients with chronic diseases.
II. Related Art
Current classification systems include those formulated by the New York Heart Association (NYHA) and by Dr. Karl Weber. The NYHA system places patients in one of four categories based on how much they are limited during physical activity:
ClassPatient SymptomsClass I (Mild)No limitation of physical activity. Ordinaryphysical activity does not cause undue fatigue,palpitation, or dyspnea (shortness of breath).Class II (Mild)Slight limitation of physical activity. Comfortableat rest, but ordinary physical activity results infatigue, palpitation, or dyspnea.Class IIIMarked limitation of physical activity. Comfortable(Moderate)at rest, but less than ordinary activity causesfatigue, palpitation, or dyspnea.Class IV(Unable to carry out any physical activity without(Severe)discomfort. Symptoms of cardiac insufficiency atrest. If any physical activity is undertaken,discomfort is increased.
The Weber classification system is a well established method for categorizing patients into four classes according to peak oxygen consumption or anaerobic threshold.
A major shortcoming of the NYHA system is that it relies on subjective observations by the patient and interpretation of those observations by the physician. Recent scientific literature has identified several flaws in the Weber system, including, for example:                (1) Peak VO2 may be lower than maximally possible (does not represent a true max). A. Gitt, Circulation 2002; 106: p 3079-3084        (2) Sub maximal parameters are more practical than peak VO2, and a more appropriate predictive index. M. Hollenberg, Journal of American College of Cardiology; 2000; 36: p 197-201        
Traditionally, maximal cardiopulmonary exercise testing is performed in patients with heart failure as well as other chronic diseases to estimate functional capacity, test for ischemia and to follow general health status. Cardiopulmonary exercise testing is also used in this population clinically to follow response to treatment such as adding new medications, titrating medications, or device therapy. This form of testing is expensive and requires a medical team including MD supervision, RN or exercise specialists, along with a technician to perform the exercise studies. In addition, the equipment necessary includes a number of independent devices including an EKG system which is often integrated into a treadmill or stationary bike, metabolic cart, and a separate oximetry system. Maximal exercise testing is also a test that patients don't look forward to performing, and with heavy exercise there are increased risks.
There is a wealth of literature demonstrating the prognostic value of cardiopulmonary exercise testing, primarily in patients diagnosed with heart failure. (1) Several variables have demonstrated prognostic value including aerobic capacity (2), ventilatory efficiency (3, 4), end tidal carbon dioxide (5) and heart rate recovery (6). While the value of information garnered from this assessment technique is clear, clinical interpretation is presently cumbersome, limiting utilization of the cardiopulmonary exercise test. A formula that included all relevant exercise test variables, appropriately weighted according to prognostic value, and generating a single score would certainly improve clinical interpretation.
The importance of using a multiparametric approach to improving risk stratification has been reported in the literature (7). This article, however, only provides, the receiver operating characteristic curves of three sequential multivariate proportional hazard models. No means are provided to utilize this information in a clinical setting—only the ROC curves are provided, leaving it to the physician to interpret the meaning of multiple CPX, neurohormonal, and echo measurements.
Previously, cardiopulmonary measurements have been made using discrete stages (e.g. Bruce protocol) or ramped protocols that continue until patient symptoms (exhaustion) occur, at which point the test is terminated. The present invention contemplates a simple three-step test (rest, exercise, recovery) which makes use of resting values, average values of exercise measurements, and their difference for multiparametric consideration.
An earlier method used the scientific literature (single source) derived mean value, standard deviation, and a normalizing value (NV), (“the number of Standard Deviations used to define the normal distribution) to calculate a variable called Autononic Balance Index. The NV was used to calculate an ABI coin and the NV was usually set to 2, since this is the classically defined definition of the “normal” range of values for a population measurements”.